In Python 2.6, I used matplotlib to make some simple graphs. However, it is incompatible with Python 3.1.
What are some alternative modules that can accomplish the same thing without being very complex?
You say you want to create some simple graphs but haven't really said how simple or what sort of graphs you want. So long as they aren't too complex you might want to consider using the Google Chart API.
That has some advantages: you just have to construct a URL that describes the desired chart so there shouldn't be any issues using it from Python 3.x. Of course there are disadvantages also: you need to have an internet connection when generating the chart, and you might not have the chart styles you have been using with matplotlib.
If you don't want to construct the URLs directly there is at least one Python wrapper for the charts API. It doesn't work directly on Python 3.x, but running it through 2to3 seems to convert it successfully.